The invention relates in general to semiconductors and in particular to a new and useful semiconductor component for the conversion of light to electric energy having at least one layer of amorphous silicon and a cover layer of polycrystalline silicon and an anti-reflection layer.
In German patent application No. P 29 38 260.3; U.S. application Ser. No. 188,725 filed Sept. 19, 1980, and now abandoned, it is described how, by the use of a cover layer of polycrystalline silicon on the semiconductor layer of amorphous silicon, the blue yield of solar cells of p-i-n structure and hence the efficiency can be substantially improved. It was pointed out in this patent application that a thin poly-crystalline silicon layer is useful in an a-Si cell to help eliminate the light absorption in the highly doped silicon contact layer on the light incidence side.
In optimizing the efficiency of solar cells, it is important, besides improving the internal collector efficiency, i.e. of the electric charge carriers produced by light absorption, to find ways to let the incident light penetrate into the active cell material without reflection losses.
According to the prior art, see e.g. U.S. Pat. No. 4,064,521 column 3, lines 38 ff, the reflection losses are minimized by applying (transparent) anti-reflection layers, (see FIG. 1) in which 1 denotes a cover plate, 2 the anti-reflection layer, and 3 the layer of a-Silicon (Si).
In the simplest case, when the reflecting extinction is to take place only in a limited spectral region and economically, the anti-reflection layer 2 has the optical layer thickness of a quarter wavelength (.lambda./4), where for extinction the refractive index n.sub.AR of the anti-reflection layer depends on the refractive indices of the adjacent media 1 and 2 according to the equation n.sub.AR =.sqroot.n.sub.1 n.sub.2.
For semiconductor components of crystalline Si (with n.sub.x-Si approx. 4 at 6000 A one uses as the material for the anti-reflection layer SiO, TiO.sub.x (n=2.3), Ta.sub.2 O.sub.5 (n=2.05), tin oxide (n approx. 2), and indium tin oxide (n approx. 2).
For semiconductor components based on amorphous silicon (a-Si), which have become of great interest recently because of the possibility of cheap manufacture, usually indium tin oxide (ITO) has been used as an anti-reflection layer. Just as SnO.sub.2, ITO offers the further advantage that due to its high electric conductivity (transparent semiconductor) it contributes greatly to the reduction of the electric layer resistance on the light incidence side in a-Si cells. Therefor, electrode grid structures of greater inter grid spacing become possible.
Now both indium tin oxide and tin oxide are suitable for reduction of the reflection loss on crystalline Si; however, when used on a-Si there remains a residual reflection R=10% in the green region of the spectrum (FIG. 1), as the refractive index of a-Si with .perspectiveto.5 is substantially higher than that of crystalline Si. For minimized the residual reflection on a-Si, an anti-reflection layer with refractive index n.sub.AR=.sqroot.n.sub.glass xn.sub.a-Si .perspectiveto.2.8 would be necessary, for which one could use for example TiO.sub.x with x.perspectiveto.2. TiO.sub.2 and also other transparent materials of comparably high refractive index have, however, a low electric conductivity and therefore cannot make the desired contribution to the reduction of the layer resistance.